At present, a metal sheet such as electrotinplate, tin free steel (TFS) and aluminum are widely used for can stock after coating with lacquer one or several times. The lacquer coating is disadvantageous from the standpoint of energy cost because a long time is required for curing the lacquer and a large volume of solvent discharged during lacquer curing must be burned in another furnace in order to prevent air pollution.
Recently, the lamination of thermoplastic resin film on a metal sheet was attempted in order to avoid these problems. For example, the methods shown in U.S. Pat. No. 3,679,513, Japanese Patent Publication No. Sho 60-47103, Laid-Open Japanese Patent Application No. Hei 3-212433, Japanese Patent Publication No. Sho 63-13829 and U.S. Pat. No. 4,957,820 are already known.
U.S. Pat. No. 3,679,513 relates to a method for lamination of a polyethylene film on a steel sheet covered with double layer consisting of a lower layer of metallic chromium and an upper layer of hydrated chromium oxide; namely TFS, which has been heated to above the melting temperature of the polyethylene film and thereafter immediately quenching the laminate. However, when the polyethylene film laminated metal sheet according to this patent is treated by hot steam at 100.degree. to 130.degree. C. in a retort, the surface of the laminated polyethylene film is partially melted by contacting with hot steam and hot water. As the result, the appearance of the laminate becomes poor. Therefore, the laminate according to this patent is not suitable for the outside of the food cans treated with hot steam in a retort for sterilization after packing food such as coffee drink, meat or fish, although the employed polyethylene film is cheaper than the polyester resin film described below. Japanese Patent Publication No. Sho 60-47103 relates to a process for lamination of a crystalline polyester resin film to a metal sheet heated to above the melting temperature of the polyester resin film and thereafter immediately quenching the laminate. In this patent, the crystalline polyester resin film is sufficiently adhered to the metal sheet by an amorphous non-oriented polyester resin layer which is formed at the interface of the crystalline polyester resin film and the metal sheet as a result of the heating step. However, when the polyester resin film laminated metal sheet according to this patent is treated by hot steam and hot water at 100.degree. to 130.degree. C. in a retort, the surface of the polyester resin film laminated metal sheet becomes milky. Especially, the part of the surface of the polyester resin film laminated metal sheet, where the hot water droplets accumulate, becomes noticeably milky. The reason for the discoloration of the surface appearance described above is due to the recrystallization of the amorphous non-oriented polyester resin layer by contact with hot steam and hot water. Therefore, the laminate according to this patent is not suitable for the outside of the can treated with hot steam in a retort for the sterilization of the packed foods.
The method according to Laid-Open Japanese Patent Application No. Hei 3-212433 is characterized by laminating a copolyester resin film having specified characteristics at a temperature below the melting temperature and above the softening temperature of the copolyester resin film in order to decrease the amount of the amorphous non-oriented copolyester resin layer formed as a result of heating step during lamination which causes the milky change during retort treatment as described above.
A small amount of the amorphous non-oriented copolyester layer is inevitably formed between the surface of the metal sheet and the biaxially oriented copolyester resin layer. If the amorphous non-oriented layer is not formed at al, the laminated biaxially oriented copolyester resin film will not adhere to the metal sheet. Thus, if the milky change is eliminated, the laminated copolyester resin film may peel off during forming operations, because the amorphous non-oriented copolyester resin layer is scant and non-uniform.
According to JPA Hei 3-212433, then, it is very difficult to obtain a metal sheet laminated with copolyester resin film which exhibits good adhesion and formability and does not exhibit a milky change during retort treatment. Furthermore, the biaxially oriented copolyester resin film sued int his patent is expensive compared with the polyethylene film used int eh present invention.
The surface of the polyester resin film laminated metal sheet according to Japanese Patent Publication No. Sho 63-13829 does not become milky by contact with hot steam and hot water in a retort, because an amorphous non-oriented polyester resin layer is almost not present as a result of lamination at a temperature below the melting temperature of the polyester resin film. However, the coating and curing of the specified adhesive precoated to the polyester resin film which is indispensable in this patent is disadvantageous from the standpoints of the material cost and the treatment of a large volume of solvent discharged during lacquer curing for the prevention of air pollution.
U.S. Pat. Nos. 4,957,820, 3,679,513 relates to a laminate wherein one side of a metal sheet is laminated with a composite polyester resin film consisting of an inner layer of non-crystalline polyester and an outer of a linear polyester, preferably biaxially oriented polyester and the other side of a metal sheet is laminated with a polyolefin-containing film comprising a bonding resin which is an acid modified polyolefin resin containing carboxyl or anhydride group. If the polyester resin film laminated side in the laminate according to this patent is used for the outside of the can treated with hot steam and hot water in a retort after packing foods, the surface of the laminated polyester resin film changes to milky by a retort treatment, because the amorphous non-oriented (non-crystalline) polyester resin layer which constitutes an inner layer in the laminated polyester resin film is recrystallized by a retort treatment as same as that in Japanese Patent Publication No. Sho 60-47103. Furthermore, if the polyolefin resin film laminated side in the laminate according to this patent is also used for the outside of the can treated with hot steam and hot water in a retort, the surface of the laminate may not change by a retort treatment. However, the polyolefin film must contain a special bonding resin. Therefore, the laminate according to this patent is not suitable for the outside of the can treated with hot steam in a retort from the standpoints of the production costs of the polyolefin film and the polyester resin film and the resistance to hot steam and hot water.